Plasma processes are widely used, for example, in semiconductor manufacturing to implant wafers with various dopants, to deposit or to etch. In order to achieve predictable and repeatable process results, it is critical to closely monitor and control the plasma characteristics. For example, plasma processes inherently produce ionic and neutral species. In a plasma doping (PLAD) process, such ionic and neutral species may react and deposit on surfaces such as the walls of the process chamber and the workpiece to be treated. Such ionic and neutral species may also react and etch such surfaces. In addition, studies of PLAD processes have shown that ion composition of a plasma may be a critical piece of information that determines dopant species, dopant depth profiles, process-related contamination, etc. The ion composition changes with PLAD process parameters such as gas ratio, total gas pressure, and discharge power. The ion composition can also change significantly depending on the conditioning status of a plasma chamber. Therefore, it is important to know the ion composition during a PLAD process, preferably in situ and in real-time, in order to achieve repeatable and predictable process results.
One conventional method of monitoring plasma conditions includes optical diagnostic techniques such as optical emission spectroscopy to monitor plasma constituents. However, a drawback with such optical diagnostic techniques is that they require transparent optical input and viewing ports. The transparency of these ports tends to degrade over time as deposits build up on the same. Another conventional method of monitoring plasma conditions includes residual gas analyzers (RGA) and mass spectrometers such as a time-of-flight mass spectrometer to monitor the plasma. However, a drawback to RGAs and mass spectrometers is that they typically require two to three orders magnitude of a lower vacuum environment than the pressure in a typical plasma processing chamber. This would then require differential pumping to achieve the desired pressure and the possibility of reactions between the ions and neutrals before they are analyzed can degrade the monitored results. In addition, the mass spectrometers tend to be bulky and may therefore perturb plasma under measurement which could distort process results. The bulkiness of mass spectrometers may also limit their deployment locations in a semiconductor process tool. In addition, a time-of-flight mass spectrometer does not ionize neutral particles so it does not monitor the same. Furthermore, a time-of-flight sensor can also not distinguish between two ions having the same mass which can further degrade the monitored results.
Accordingly, it would be desirable to provide a technique for monitoring a plasma process with an ion mobility spectrometer which overcomes the above-described inadequacies and shortcomings.